Color autostereoscopic display

ABSTRACT

A polychromatic three dimensional display comprises a first (5) and second (8) image sources, the second image source (8) adapted to reduce selectively the field of view of the first image source to provide thereby a time multiplexed three dimensional autostereoscopic image. The display also comprises a switching color filter (12) disposed adjacent to the second image source which comprises a plurality of regions each switchable between different colors to enable color modulation of the generated image.

BACKGROUND OF THE INVENTION

The present invention relates to three dimensional autostereoscopicdisplays. Such autostereoscopic displays can be made from a high framerate two dimensional display and a device which makes the picture on thetwo dimensional display visible from different directions.

To display an autostereoscopic three dimensional image, a series ofviews of the object to be imaged are required. These might be capturedby, for example, surrounding a solid object with an array ofconventional cameras.

With such systems, each view in the series is put up on the twodimensional display in turn and made visible from a particular generaldirection. If the series is repeated quickly enough that the human eyeperceives no flicker, the apparent effect is a display whose imagecontent will depend on from where the human eye looks. By appropriatematching of view to direction of viewability, it is possible to recreatethe three dimensional image on the display.

One way of making such a display is to use a cathode ray tube as the twodimensional display, and a lens and a shutter as the device which limitsthe field of view of the picture on the display.

The lens forms a virtual image of the cathode ray tube. The shutter,which reduces the field of view of the image, is positioned adjacent tothe lens.

If the position of the aperture in the shutter can be changed rapidly,the angle from which the image can be seen may be varied as differentpictures are presented in turn for display on the CRT screen. Eachpicture can be the view of some scene taken from a chosen viewpoint. Solong as the picture for each direction is repeated sufficientlyfrequently, typically at least 50 Hz, and the shutter is stepped insequence with the view on the CRT display, then different views will beseen from different positions and a three dimensional image will beobserved. There are several alternative optical and image formingarrangements that are operationally equivalent to the description givenabove. Implementations can consist of 2 dimensional image formingdevices made from an LCD or from a CRT. The shutter can be made from anLCD. The arrangement can place the shutter between the image formingdevice and the observer or can position the image forming device betweenthe shutter and the observer. The light can be collimated ornon-collimated. In a presently preferred implementation, the imageforming device is a CRT and a viewing lens is added between the viewerand the shutter to narrow the pencils of rays passing through theaperture into near-parallel beams.

A number of devices for producing a colour 3D display employing theabove principles have been proposed. One such way of making a colourthree dimensional display is to have a colour CRT. However conventionalcolour CRT's are not bright because they incorporate shadow masks. Sincethe shuttering system absorbs a lot of light the CRT in this systemneeds to be bright, requiring a great deal of power, and brighter thanavailable by current masking techniques.

Another way of making a colour three dimensional display is to usedichroic mirrors to combine the images from one red, one green and oneblue CRT. The problem is that, at least in current three dimensionaldisplays, the imaging lens is large, and there is little space fordichroic mirrors. Furthermore, it becomes necessary to register the CRTsso that the position of each pixel is the same on each CRT. The wholesystem is bulky, heavy and expensive because three times as manycomponents are needed.

The bulk and expense of three CRT's can be avoided by using one CRT witha white phosphor, then filtering the emitted light so that rays of eachof the primary colours are transmitted in turn. The disadvantages ofthis approach are that the CRT needs to operate three times faster thanotherwise, it needs to be brighter than otherwise, and a filter whichcan switch between the primary colours is required.

A filter which switches between the primary colours has been made for acolour two dimensional display. It might seem that the obvious way tomake a colour three dimensional display would be to place this filterover the front of the three dimensional display. Colour images might beproduced on a black/white three dimensional display by placing a filteragainst the front which switches through the primary colours. Theproblem is that the liquid crystal of which present switching colourfilters are made cannot switch quickly enough.

When the filter is used on a two dimensional display the CRT displaysthe three primary colour components of the picture one by one. Ideallythe filter should change colour in the time interval between the end ofthe display of one primary colour and the beginning of the display ofthe next. This time interval is short and even on the two dimensionaldisplay the filter is not fast enough for this purpose.

In fact the filter is divided into horizontal bands which can beswitched independently. The idea is that as the CRT electron beam writeseach picture from the top of the screen to the bottom, the horizontalbands of the filter are also switched one by one, from top to bottom.Provided each band begins switching immediately after lines adjacent toit have decayed, there is sufficient time to complete switching beforethe adjacent lines are written with new information.

This solution works for the two dimensional display, but the frame rateof the CRT in the three dimensional display is much higher. There istherefore less time before each line is rewritten, and this time isinsufficient for the filter to switch.

An alternative to using a switching colour filter would be to make awheel comprising a red, green and blue filter and spin this in front ofthe screen. This might work with a two dimensional display because thewheel need only spin at 60 Hz. With a three dimensional display with 8views, for example, the wheel would have to spin at 480 Hz, this wouldbe impractical.

The presently available switching colour filters are slow because theyare based on slow-switching liquid crystals. The liquid crystals in theshutter used in the three dimensional display switch much more quickly.It has been suggested that it might be possible to configure the shutterto switch each slit through each of the primary colours in turn beforeclosing that slit and opening the next. Colour images might be producedon a black/white three dimensional display by making the shutter filterthe primary colours in turn while each slit is open. Unfortunately,shutters which work in this way are not available at present.

It is useful to consider if a pair of spinning wheels could beconfigured to behave like a colour-switching shutter. Such a system isdiscussed in "Wireless World, February 1942--Stereoscopic colour TV". Ina system with a wheel with a slit spinning so as to scan the slit in thefocal plane of the collimating lens, with a colour wheel placed adjacentto this slit, the colour wheel spinning at a higher rate so that at eachslit position light is filtered to produce each of the primary colours.

The problem with this proposed system is that the shutter slit does notmove from position to position, but is always moving. So the slit willbe at a slightly different position as each of the colour filters passesit. This will produce a registration problem, there will be positions atwhich an eye will see, for example, the red component of one viewsuperposed on the blue and green components of a different view. Thiswould be quite unsatisfactory.

SUMMARY OF THE INVENTION

According to the present invention there is provided a polychromaticthree dimensional display comprising:

a first image source;

a second image source, the second image source being adapted to reduceselectively the field of view of the first image source to generatethereby a time multiplexed three dimensional autostereoscopic image; and

a switching colour filter disposed adjacent to the second image sourceand comprising a plurality of regions each switchable between differentcolours to enable colour modulation of the generated image.

Preferably the first image source is a spatial light modulator, and thesecond image source is a scanning light source which selectivelyilluminates the first image source from one of a plurality of positions.Alternatively, the first image source is a monochrome or white phosphorcathode ray tube or similar device, and the second image source is ashutter comprising a plurality of independently activated apertures.

Preferably, there is also provided an imaging lens positioned betweenthe two image sources. The imaging lens may be either single ormulti-element, and allows greater optical design freedom, together withthe possibility of producing large images from components of reducedsize.

A collimating lens may also be provided to provide a viewer withcollimated light to improve the autostereoscopic effect of the display.

The filter may include a rotatable disc comprising a plurality ofdifferently coloured regions, but is preferably comprises a plurality ofregions that are individually switchable between a plurality of colours.With this latter arrangement, for displays where the second image sourceis a shutter, there may also be provided control means for controllingthe activation of the switching colour filter strips and shutterapertures so that each strip starts switching to the next colourimmediately after the termination of the view which passes light throughthat strip.

Preferably the colour filter has portions corresponding the threeprimary colours.

BRIEF DESCRIPTION OF THE DRAWINGS

One example of the present invention will now be described withreference to the accompanying drawings in which:

FIG. 1 is a schematic diagram showing the basic principles of anautostereoscopic three dimensional display;

FIG. 2 is a diagram showing a prior art example of a monochromeautostereoscopic three dimensional display;

FIG. 3 is a colour adaptation of the display of FIG. 2;

FIG. 4 is a prior art polychromatic autostereoscopic three dimensionaldisplay employing a spinning disc;

FIG. 5 is a schematic diagram of a first example autostereoscopicpolychromatic three dimensional display according to the presentinvention;

FIG. 6 is a schematic diagram of a second example autostereoscopicpolychromatic three dimensional display according to the presentinvention; and

FIG. 7 is a schematic diagram of a third example autostereoscopicpolychromatic three dimensional display according to the presentinvention.

DESCRIPTION OF THE INVENTION

FIG. 1 shows the basic concept of a known three dimensional display.Pictures of an object are formed by cameras 3 ranged round the object 2and pointing at it from different directions. One picture at a time isreproduced on the display 1. The display 1 can confine the directionfrom which this picture is visible. It does this so that the directionof visibility matches the direction of the camera from which the pictureis received. Other pictures are shown from other cameras 3 in a similarmanner. Once a picture from each of the cameras 3 has been shown, thesequence is repeated. The rate of repetition is such that the display ofeach picture to each direction will appear continuous to an observer 4inspecting the display from different angles.

Whenever collimated light illuminates the observer 4, he will see thepicture on the display. However each of the observer's eyes will beilluminated by collimated light travelling in different directions. Soeach eye will see a different picture. The three dimensional imagedisplayed will therefore be of the type described with reference to FIG.1.

In the prior art system of FIG. 2, a cathode ray tube (CRT) 5 is used asan image source. Light from the image source 5 passes through an imaginglens 6 and an aperture 7 in a shutter 8. The shutter 8 comprises anarray of independently activated apertures 7. The light then passesthrough a collimating lens 9 to be viewed by an observer. A differentaperture 7 is opened for each of the images that are displayed on theimage source 5, so that a viewer perceives each of the images to be froma source of a different position and a monochrome three dimensionaldisplay is produced.

FIG. 3 shows the monochrome three dimensional display of FIG. 2 adaptedto be used a colour three dimensional display by the additional of acolour filter 10. The colour filter 10 switches between the threeprimary colours in turn, and the sequence of images displayed in theimage source 5 is repeated three times, once for each of the primarycolours. As mentioned above, such a system has the serious problem thata switching times of the image source 5, shutter 8 and particularly thecolour filter 10 must be very small so that no flicker is observed bythe viewer.

In the prior art system of FIG. 4, the moving slit shutter 8 is providedby a spinning disc, and the colour is produced by the rotation of aspinning disc 11 that is divided into three portions, each of theportions being coloured according to one of the three primary colours.In this example, the filter disc 11 is spun at high speed to producethree supposedly identical images in the three primary colours, whichare perceived by an observer to be a single image of combined colour.Unfortunately, as the slit 7 is continuously moving, the three colouredimages will be perceived to be coming from slightly different positions,so that they will not overlap perfectly and a full three dimensionaleffect will not be produced.

In the first example of the present invention shown in FIG. 5, thedisplay has a first image source 5, which is a spatial light modulatorprovided by a liquid crystal display or similar device. The device alsoincludes a scanning light source 8 and spinning disk filter 12. There isalso provided an imaging lens 6, which is not essential, but whichprovides greater component design freedom. In operation the first imagesource 5 displays a series of images of an object from differentviewpoints, and, for each image displayed, a different portion of thelight source is activated, illuminating the image from one direction andmaking it appear to come from a different position. As with the priorart examples, with each of the images being produced at a rate at whichthe eye perceives no flicker, an autostereoscopic display is produced.However, as there is provided a colour filter 12 between the lightsource 8 and image source 5, a series of colour modulated images will bepresented to a viewer. The colour filter 12 must spin at a speed whichensures that the illuminating light is modulated to the correct colourfor the image being presented, but as only one of the light sources 8 isactivated at any one instant, the rotation of the colour filter 12 canfollow the activation of the individual light sources 8, providing acomplete series of images for a first colour, and enabling the filter 12to have rotated to the next colour by the time that each of the sources8 has been activated. This greatly reduces the necessary spinning speedfor the filter.

FIG. 6 shows a second example of the present invention which employsvery similar principles to that of the first example, but which, inplace of the spinning colour filter 12 has a colour filter comprising aplurality of individually switchable regions 13. Each of these regions13 can be switched between one of a number of colours, in this examplered, green and blue. An example of a device with such characteristics isa NU 700S colour shutter from Tectronix Ltd. In this example, the lightsource 8 and image source 5 operate in a similar fashion to that of thefirst example, but the filter is aligned with the individual lightsources. In operation, each of the regions 13 of the filter 12 isactivated to change colour immediately after its corresponding lightsource has been de-activated, so that the time period in which eachregion must change to the next required colour is maximised. Thisenables the employment of a filter with a reduced switching speed foreach of its regions.

In the example of the present invention shown in FIG. 7, a first imagesource 5, imaging lens 6, shutter 8 and collimating lens 9 are provided,in addition, a switching colour filter 12 is also provided. The imaginglens 6 and collimating lens 9 are not essential to the invention, but,as mentioned above, enable greater design freedom and components ofreduced size to be employed. The switching colour filter 12 ispositioned between the imaging lens 6 and the shutter 8 and, as with thesecond example, comprises an array of individually switchable regions13, each of the regions being able to be switched between the primarycolours. This example may, alternatively, employ a spinning disc filterof the type described with reference to FIG. 5, in place of theswitchable strip colour filter 12.

As previously mentioned, spinning disc colour filters would normallyhave to be spun at great speed to be employed in an autostereoscopicdisplay, but, with the examples of the present invention which employsuch filters, this speed is greatly reduced by the employment of only afraction of one of the coloured apertures in colour modulation at aparticular instant. Also, as previously mentioned, the switching time ofswitchable colour filters is slow, but in the two examples of thepresent invention which employ such filters, this is overcome byindividually switchable strips 13, which can be activated prior to theircorresponding aperture 7 being opened in front of them. As each of thestrips is only visible for a short period of time, a larger switchingtime is available for activating them and changing their colour.Different speed of switching colour and aperture can be exploited togive the combined effect on the two at the speed of the fastest, subjectto a cycle time of the switching speed of the slowest. The examples ofthe present invention operate in a similar fashion to the device of FIG.3, in that the image sequence is run three times on the image source 5,with each of the apertures 7 being activated in turn on the shutter 8once for each time the sequence is played, the filter 12 being switchedbetween colours in advance of the opening of the aperture 7 so that ithas completely changed to the next colour prior to them being madevisible to the viewer.

We claim:
 1. A polychromatic three dimensional display comprising:afirst image source operating at a first selected rate; a second imagesource switching through a repeating cycle at a second selected rateless than the first selected rate to reduce selectively the field ofview of the first image source to generate thereby a time multiplexedthree dimensional autostereoscopic image; and a switching colour filteroperating at the second rate disposed adjacent to the second imagesource and comprising a plurality of regions each switchable betweendifferent colours to enable colour modulation of the generated image. 2.A display according to claim 1, wherein the first image source is aspatial light modulator, and the second image source is a scanning lightsource which selectively illuminates the first image source from one ofa plurality of positions.
 3. A display according to claim 1, wherein thefirst image source is a white phosphor cathode ray tube or similardevice, and the second image source is a shutter comprising a pluralityof individually activated apertures.
 4. A display according to claim 1,wherein there is further provided an imaging lens positioned between thetwo image sources.
 5. A display according to claim 4, wherein theimaging lens is a single element lens.
 6. A display according to claim4, wherein the imaging lens is a multi-element lens.
 7. A displayaccording to claim 1, wherein there is further provided a collimatinglens to provide a viewer with collimated light.
 8. A display accordingto claim 1, wherein the filter includes a rotatable disc having aplurality of differently coloured regions.
 9. A display according toclaim 1, wherein the filter comprises a plurality of regions that areindividually switchable between a plurality of colours.
 10. A displayaccording to claim 9, wherein each of the regions are rectangular andadjacent to one another.
 11. A display according to claim 1, whereinthere is further provided control means for controlling the activationof the switching colour filter strips and shutter apertures so that eachstrip starts switching to the next colour immediately after thetermination of the view which passes light through that strip.
 12. Adisplay according to claim 1, wherein the filter has portionscorresponding to the three primary colours.
 13. A display according toclaim 1, wherein the first image source displays real two dimensionalimages.